The present invention relates to a pressure sensor assembly and to a method of using the assembly.
Pressure gauges are widely used in all segments of the industry including exploration and production of hydrocarbons, and in particular for use downhole in oil and gas wells. Pressure gauges are typically constructed using piezoelectric, ferroelectric and resonant quartz components that are active to pressure and temperature characteristics.
Fiber optic pressure gauges have become available over the last decade.
Fiber optic gauges have a number of distinct advantages over electrical gauges in the sense that fiber optic gauges are refrained from on-board electronics, unaffected by electromagnetic interference and inherently intrinsically safe. As such, fiber optic gauges are often used in explosive or flammable environments, and for use at high ambient temperatures, like in oil and gas wells.
U.S. Pat. No. 6,233,746 discloses a solution of a fiber optic sensor using fibre Bragg gratings whereby a single optical fiber extends through the sensor housing, and whereby a grating for measuring pressure and a grating for measuring temperature are closely spaced along the optical fiber. The pressure grating is bonded to a diaphragm and the temperature grating is floating in a cavity inside the sensor housing.
International patent application WO2006097772 discloses a sensor system for use in a well bore, which includes a metal-clad fiber optic cable. The fiber cable includes one or more Fiber Bragg Gratings (FBGs) one of which is attached to the centre of a pressure sensing diaphragm, such that when the diaphragm is subjected to pressure the FBG is strained and a change in wavelength reflected from the FBG is measured. The fiber in the region of the second FBG is not attached to anything but is free standing or attached loosely to an object in the chamber and its reflection wavelength is only affected by temperature, and is used to correct for temperature effects on the pressure FBG. If the fiber is coated with a polymer such as polyimide or similar polymer the fiber is bonded to the diaphragm using low viscosity high temperature adhesive, if the fiber is copper coated, it can be attached to the diaphragm by alternative methods, which increase its range of temperature operation significantly.
International patent application WO2006097772 discloses in FIGS. 15 to 17 an absolute pressure and temperature sensor utilising FBG optics. One FBG element is bonded to the outer surface of the housing and the other is bonded to a pressure sensitive disc, the fiber being allowed to follow a helical path, so that when the sensor is assembled no unnecessary stress is produced in the fiber. In the atmospheric chamber, one FBG sensor provides a reference measurement temperature, and the other FBG sensor, which is attached to the pressure-sensing disc, provides a measurement for temperature together with the applied pressure. If the reference measurement is deducted from the combined measurement, then a true measure of absolute pressure can be determined.
International patent application WO2006097772 discloses in FIG. 27 that a FBG fiber includes a first FBG bonded to a first pressure diaphragm in contact with one region of the well via a port, before a second FBG in the fiber is bonded to a piece of aluminium, and finally a third FBG is bonded to a second pressure diaphragm in contact with a second region of the well via a second port in order to provide a differential pressure value as previously described. The piece of aluminium quickly follows any change in ambient temperature, so the second FBG provides a value by which changes in the readings of the first and third FBG, which are due to temperature, rather than changes in pressure can be compensated for. It will be seen that in this embodiment the path of the fiber is approximately linear as it passes through each sensor and on to the next, as opposed to sensor shown in FIGS. 15 to 17 where the fiber is coiled so as to enter and exit through the sensor's single opening.
International patent application WO2007003445 discloses a meter system for measuring parameters of conditions within a production tubing, comprising a plurality of meters being linked by a single fiber-optic cable, and a membrane with two parts, which when assembled sandwiches a Fiber Bragg Grating or FBG, which is embedded in the cable. Two FBGs are required, the first FBG measures both strain caused by pressure and temperature, and the second FBG measures the temperature, and differential equations allow the pressure value to be determined.
A fiber optic cable incorporating two FBG sensing elements is fed into the tube and one FBG element is bonded to the outer surface of the housing and the other is bonded to a pressure sensitive disc, the fiber is allowed to follow a helical path, which when assembled does not create any unnecessary stress in the fiber. One FBG sensor provides a reference measurement for ambient conditions, and the FBG sensor attached to the pressure-sensing disc provides a measurement for ambient conditions together with the applied pressure. If the reference stain is deducted from the combined measurement, then a true measure of absolute pressure can be determined.
FIGS. 27 to 29 of this prior art reference disclose an absolute pressure and temperature sensor utilising FBGs sensors attached to a tube. A housing has a chamber machined in it. The chamber is closed by an end disc. A FBG is bonded to an inside surface of the housing, this measures the external pressure. A second FBG is bonded to another internal surface and this measures the internal pressure of the tube. A further FBG is bonded to a non-strained surface of the housing, and this just measures the effects of temperature. This is then used to correct the two pressure measurements.
US patent application 2002/0154860 discusses using a second FBG for temp sensing and in some embodiments this is bonded to a similar substrate that is pressure isolated. However the rest of the design is substantially different to the present invention, as the pressure sensing FBG is not directly bonded to a diaphragm.
US patent application 2004/0036005 shows a second FBG on a fiber for temp compensation with no further details or preferred embodiments of the temperature compensation sensor.
The methods and systems disclosed in the foregoing prior art references above have the following shortcomings.
A shortcoming of the method disclosed in U.S. Pat. No. 6,233,746 is that placement of the temperature grating at close proximity to the pressure grating along the same optical fiber, whereby the temperature FBG is not attached to a supporting mechanism, makes the temperature grating susceptible/very sensitive to induced strain caused by the movements of the diaphragm and/or vibration or shock of the sensor housing. For instance, when the diaphragm moves the fiber moves and these movements are directly coupled in to the temperature grating. The measuring data disclosed in U.S. Pat. No. 6,233,746 suggest that there is cross coupling from the pressure to the temperature grating in various configurations. The disclosed scheme in U.S. Pat. No. 6,233,746 would most likely also have significant pressure induced wavelength shift on the temperature grating.
A shortcoming of the methods disclosed in U.S. Pat. No. 6,233,746 and in International patent applications WO2006097772 and WO2007003445 is that placement of the temperature grating at close proximity to the pressure grating along the same optical fiber prevents the use of an improved temperature-compensating scheme that is part of the present invention based on nominally identical wavelengths for the pressure and the temperature FBG since nominally identical wavelength FBGs can only be read on the same fibre using interrogator apparatus based on time division multiplexing and such interrogator apparatus typically requires a mutual distance between the pressure and the temperature FBG of at least 1 meter. Using the sensing system as disclosed in U.S. Pat. No. 6,233,746, and in international patent applications WO2006097772 and WO2007003445 with the improved temperature-compensating scheme based on nominally identical wavelengths requires extra fiber which must be coiled inside the sensor housing, which may cause optical bend-losses in the fiber affecting the measurement, and/or a significant over-dimension of the sensor housing to an extent that it may become too large to fit downhole in a well.
Another shortcoming of the method known from U.S. Pat. No. 6,233,746 is that the proposed temperature measurement mechanism is of a lower precision than the mechanism of the present invention and so leads to relatively greater measurement uncertainty.
It is an object of the present invention to provide a pressure sensor assembly and method in which the above shortcomings are reduced or solved.